Determining optical fiber types

ABSTRACT

In determining the kind to which an unknown optical fiber ( 1, 1′ ) belongs an automatic fiber fusion-type splicer is used having movable clamps ( 21 ), electrodes ( 3 ), camera devices ( 9 ) and background illumination ( 11 ), all coupled to electronic circuits ( 25 ) containing control means ( 33 ) and driver and interface circuits ( 29, 27, 31, 28 ). A portion of the fiber is imaged on the light sensitive areas of the cameras through high-resolving lens systems, allowing the core of the fiber to be distinguished in the captured image, both when the fiber is cold and when it is heated such as to temperatures used in fusion-splicing to issue light. From a first picture taken of the fiber in a heated state a first light intensity profile is determined in an image processing and analysis module ( 15 ) along a line substantially perpendicular to the longitudinal direction of the fiber. This profile is further analyzed by calculating the derivative of the profile and comparing the derivative to derivatives of light intensity profiles previously determined for a optical fibers of known different kinds. A second picture is taken of the cold fiber for which also a second light intensity profile can be determined. This profile is then compared to corresponding profiles previously determined for the known optical fibers. The results of the comparing operations are finally evaluated to decide the kind of the tested fiber.

TECHNICAL FIELD

[0001] The present invention relates to a method for evaluating anoptical fiber, in particular for determining the general type of opticalfiber to which a considered optical fibers belongs, to be used in anautomatic fiber splicer for automatically selecting correct splicingparameters and also to a method of setting an optical system used in asplicer.

BACKGROUND

[0002] Equipment and methods for aligning and splicing silica basedoptical fibers has been developed and improved for many years. The mostcommon method for performing an alignment of optical fibers to bespliced with an accuracy better than 0.2 micron and making accuratesplice loss estimation has comprised advanced digital image processingof magnified pictures of the splicing position before, at and after theactual splicing of the fibers, these pictures produced by a camerasystem. The design of a compact optical system, capable of giving asharp image of the spliced fibers and their cores before, during andafter the splicing which in many cases is made by a fusion process, hasbeen a critical task in developing fiber splicing machines having a highperformance, see e.g. T. Haibara, M. Matsumoto, T. Tanifuji and M.Tokuda, “Monitoring method for axis alignment of single-mode fiber andsplice loss estimation”, Optics Letters, Vol 6, No. 4, April 1983, O.Kawata, K. Hoshino, Y. Miyajima, M. Ohnishi and K. Ishihara, “A splicingand inspection technique for single-mode fibers using direct coremonitoring”, J. Ligthwave Technology, Vol. LT-2, No. 2, April 1984, andT. Katagiri, M. Tachikura and I. Sankawa. “Optical microscopeobservation method of a single mode optical fiber core for precisecore-axis alignment”, J. Ligthwave Technology, Vol. LT-2, No. 3, June1984.

[0003] In a fusion splicer equipped with a digital camera system, thefibers to be spliced or being spliced or having been spliced areconventionally illuminated by a light source, normally a LED, located ata distance behind the fibers as seen from a lens system. The lens systemis focused on some point in the fiber claddings or in the fiber coresand a magnified image of the fibers is created on a CCD matrix (ChargeCoupled Device). The electric signal from the camera is A/D converted,and the digital picture is processed in a computer system. Themeasurement data from the pictures are then used for moving fibers tothe desired accurate alignment and for estimating the splice loss.

[0004] The optical system of a fusion splicer can also be adapted toimage the hot fibers during fusion, see e.g. German patent 40 04 909 andSwedish patent application 9002725-1, filed Aug. 24, 1990. The smalldifference between emissivity of the fiber cladding and the core at hightemperatures, such as those of about 2000° C. existing in an electricarc used in the fusion process, make it possible to produce a brightimage of the core, conventionally located in the middle of the fiber.The visible and near infrared part of the emitted waves from the heatedfibers are collected and detected by the camera system. Hot images canbe used for real time processing of the fibers during fusion and formaking an accurate splice loss estimation after the splice is completed.

[0005] It is well known that core/cladding eccentricity, cleave angle,curl, fiber-end contamination and mode field diameter (MFD) mismatch arethe main reasons of fusion splicing loss. An MFD mismatch cansignificantly influence the splice loss in particular in the case wheredifferent types of fibers are spliced to each other. To produce spliceshaving a low loss made between fibers having different MFD it isnecessary to characterize the types of fibers to be splice and based onthe fiber types, select appropriate splice parameters like overlap,fusion heat and fusion time to be used in the splicing procedure, seee.g. W. Zheng, “Real time control of arc fusion for optical fibersplicing”, J. Ligthwave Technology, Vol. 11, pp. 548-553, March 1994,and W. Zheng, O. Hultén and Robert Rylander, “Erbium doped fibersplicing and splice loss estimation”, J. Ligthwave Technology, Vol. 12,pp. 430-435, March 1994. The appropriate choice of these parameters ishighly dependent on the core sizes and the refractive index profiles ofthe fibers and the refractive index differences between the core andcladding. A method for identifying fibers before fusion for automaticselection of the splice parameters is therefore of great importance formaking low loss splices of different types of fibers.

[0006] In Swedish patent application 9100979-5 for Telefonaktiebolaget LM Ericsson, inventors Ola Hultén and Wenxin Zheng, a method ofdetermining characteristics of optical fibers is disclosed, the methodincluding analyzing images of heated fibers and in particular lightintensity profiles along lines perpendicular to the fibers. The generalshape of the central peak and especially its width and height areevaluated. A mathematical method using the same basic analyzing processis disclosed in Swedish patent application 9201817-5 forTelefonaktiebolaget L M Ericsson, inventor Wenxin Zheng.

[0007] In an automatic fiber splicer the optical system for imaging anoptical fiber on some light sensitive area cannot be easily set fordifferent imaging conditions such a for producing a sharp picture of acold fiber in which the core is visible or in particular the positionand the width of the core are detectable or for producing a sharppicture of a heated optical fiber emitting light so that also in thispicture the core region is detectable. Such focusing for differentimaging conditions is generally made manually by observing the capturedimages for different focusing conditions, i.e. for different distancesbetween the object, the optical fiber, and the imaging system, primarilythe lens system.

SUMMARY

[0008] It is an object of the invention to provide a reliable method ofdeciding the type of an optical fiber.

[0009] It is another object of the invention to provide a robust,automatic method of setting an optical system for providing images of anoptical fiber in which the core of the optical fiber is visible.

[0010] In determining the kind or type to which an unknown optical fiberbelongs an automatic fiber splicer using fusion-welding is used havingmovable clamps or retainers for positioning aligning two fibers,electrodes for producing when energized an electric arc, camera devicessuch as CCD-matrices and light sources producing a backgroundillumination. These devices are all coupled to electronic circuitscontaining control means (33) and the necessary driver and interfacecircuits. A portion of the fiber held one of the clamps is imaged on thelight sensitive areas of the cameras through high-resolving lenssystems, allowing the core of the fiber to be distinguished in thecaptured image, both when the fiber is cold and when it is heated suchas to about or somewhat lower temperatures used in fusion-splicing andthen issue sufficient light for capturing images without using anybackground illumination. From a first picture taken of the fiber in aheated state a first light intensity profile along a line substantiallyperpendicular to the longitudinal direction of the fiber is determinedin an image processing and analysis module. This profile is furtheranalyzed by calculating the derivative of the profile and comparing thederivative to derivatives of light intensity profiles previouslydetermined for a optical fibers of known different kinds or types. Asecond picture is taken of the cold fiber for which a second lightintensity profile can be similarly determined. This profile is thencompared to corresponding profiles previously determined for the knownoptical fibers. The results of the comparing operations are finallyevaluated to decide the kind of the tested fiber.

[0011] In an automatic fiber splicer a correct automatic focusing fordifferent imaging conditions can be obtained by executing the followingsteps in a successive order. The distance between the optical fiber andthe optical system of the splicer is varied and pictures are taken fordifferent distances. In the pictures taken light intensity profiles aredetermined as above which are analyzed to find a measure of the apparentdiameter of the optical fiber and a measure of the apparent width of thecentral peak which is normally obtained in such profiles and correspondsto the high intensity region in the center of the fiber and which atleast for a high-resolving optical system corresponds to the core of thefiber. The ratio or quotient of these two measures is calculated andcompared to a predetermined value. The distance giving a picture inwhich the ratio of the measure values agrees with the predeterminedvalue or at least deviates as little as possible from that value istaken as the distance giving a correct imaging. It turns out that bymeasuring on fibers of different types a predetermined value can bedetermined which produces good pictures of cold pictures from whichvaluable information of the core such as its diameter can be obtainedand a different predetermined value can be determined producingcorrespondingly good pictures of heated fibers.

[0012] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the methods, processes, instrumentalities and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] While the novel features of the invention are set forth withparticularly in the appended claims, a complete understanding of theinvention, both as to organization and content, and of the above andother features thereof may be gained from and the invention will bebetter appreciated from a consideration of the following detaileddescription of non-limiting embodiments presented hereinbelow withreference to the accompanying drawings, in which:

[0014]FIG. 1a is a schematic picture of a setup for fusion-splicing twooptical fibers to each other,

[0015]FIG. 1b is a schematic picture similar to that of FIG. 1a alsoshowing some components of electronic control circuits,

[0016]FIG. 2a is a cold image of an optical fiber, when the fiber islaterally illuminated by a light source,

[0017]FIG. 2b is a hot image of an optical fiber, captured in a fusionprocess when the fiber glows and radiates light,

[0018]FIG. 3 is a diagram showing the radiation spectrum of an opticalfiber in fusion, the spectral response of a CCD-camera and the productof the radiation spectrum and the spectral response,

[0019]FIG. 4 is a diagram of the refractive index profile of adispersion shifted fiber measured by a fiber geometry scanner,

[0020]FIG. 5 is a schematic view illustrating refraction of collimatedlateral rays from a light source by an optical fiber,

[0021]FIG. 6 is schematic sectional view showing the design of ahigh-resolving lens system,

[0022]FIG. 7a is a plot of the modulation transfer function of the lenssystem of FIG. 6 for three wavelengths in the range of 610-690 nm, alsoshowing the ideal MTF (diffraction limited),

[0023]FIG. 7b is a plot similar to that of FIG. 7a for wavelengths inthe range of 500-700 nm,

[0024]FIGS. 8a and 8 b are cold and hot images of a typical dispersionshifted fiber imaged by the lens system of FIG. 6 as captured by aCCD-camera, the cold image taken when the fiber is laterally illuminatedby a red LED (λ_(c)=660 nm) and the hot image taken during fusion, whentemperature of the fiber is approximately 1900° C.,

[0025]FIG. 9 is a plot of hot and cold image profiles of the dispersionshifted fiber of FIGS. 10a, 10 b in grey scale values,

[0026]FIG. 10 is a plot of the hot image profile F(x) and its firstorder derivative G(x) for the dispersion shifted fiber of FIGS. 8a, 8 b,

[0027]FIG. 11 is flow diagram illustrating a procedure for fiberidentification,

[0028]FIG. 12a is a diagram of the refractive index profile of a regionat the core of a depressed cladding, highly doped dispersion shiftedfiber,

[0029]FIG. 12b is a diagram showing measured pixel grey scale valuesQ_(m)(x) and S_(h)(x) for cold and hot fibers respectively and thederivative array G_(h)(X) of the hot profile of the fiber of FIG. 12a,

[0030]FIGS. 12c-12 d are cold and hot images of the fiber of FIG. 12a,

[0031]FIGS. 13a-13 d are diagrams and pictures corresponding to those inFIGS. 12a-12 d but for a matched cladding, erbium doped fiber,

[0032]FIGS. 14a-14 d are diagrams and pictures corresponding to those inFIGS. 12a-12 d but for a standard single-mode fiber, SMF-28,

[0033]FIGS. 15a-15 d are diagrams and pictures corresponding to those inFIGS. 12a-12 d but for a large effective area none-zero dispersionshifted fiber,

[0034]FIGS. 16a-16 d are diagrams and pictures corresponding to those inFIGS. 12a-12 d but for a multi-mode fiber, and

[0035]FIGS. 17a-17 d are diagrams and pictures corresponding to those inFIGS. 12a-12 d but for a pure silica core single-mode fiber.

DETAILED DESCRIPTION

[0036] In FIG. 1a the basic setup in an automatic optical fiber spliceris shown which is similar to that used in a prior art ribbon fibersplicer, see U.S. Pat. No. 5,961,865. The optical fibers 1, 1′ to be orbeing spliced have their end regions located between points ofelectrodes 3, between which an electrical discharge 5 is generated forheating the fiber ends, the intensity of the electrical discharge beingcontrolled by the intensity of the electrical current between theelectrodes 3. Optical lens systems 7 depicts, in two perpendiculardirections, the fiber end regions on light sensitive areas 9, typicallyplates carrying a matrix of CCD elements. The splice position can beilluminated by suitably placed light sources such as LEDs 11 providing abackground illumination when required. A digital imaging processingsystem 15 processes the electric signals from the light sensitive areas9 to monitor the fibers used and the splicing procedure by controllingfiber positioning devices and the intensity of the electrode current.The image processing system is connected to a monitor or display element17 for showing the captured images.

[0037] In the schematic picture of FIG. 1b some more electrical detailsof a fiber splicing device of the automatic type are shown. Thus, thesplicing device has fixtures or retainers 21, in which the end portionsof the fibers 1, 1′ are placed and firmly held during the positioningand the splicing. The retainers are movable in three orthogonalcoordinate directions both in parallel to the longitudinal direction ofthe fibers and in two directions perpendicular to this direction. Theretainers 21 are thus displaced along suitable mechanical guides, notshown, by control motors 23. Electric lines to the electrodes 3, thelight sources 11 and the motors 23 extend from an electronic circuitmodule 25, from driver circuits 27, 28 and 29 respectively of thecircuit module. From the light sensitive areas 9 electric lines arearranged to a video interface 31 in the electronic circuit module 25,from which a suitable image signal is delivered to the image processingand image analysis unit 15. The various procedural steps are controlledby a control circuit 33 of the circuit module, e.g. a suitable microprocessor. The control circuit 33 performs the procedural stepsmentioned above and thus controls the displacement of the fiber ends inrelation to each other by energizing the motors 23 in suitabledisplacement directions, provides electrical current to the lightsources 11 when pictures e.g. for the alignment procedure have to becaptured, and provides a signal to the image processing and imageanalysis unit 15 for starting an analysis of an obtained image. Further,the control circuit 33 controls the time, when a fusion current is to bestarted to be provided to the electrodes 5 and the time period duringwhich this current is to be delivered and the intensity of the fusioncurrent.

[0038] In order to obtain images of cold fibers in which the fiber coresare visible and which are suited for digital processing, the lenssystems 7 used must have special performance characteristics andfeatures. Thus, the lens systems 7 must be capable of imaging the coresof the optical fibers both when the fibers are not heated and arelaterally or from behind illuminated by a separate light source to get a“cold image” and such a picture is seen in FIG. 2a of a typicalsingle-mode fiber. The lens systems 7 must also be capable of imagingthe fibers during the fusion process when the fibers are hot and emitthermal radiation to get a “hot image”, see the picture of FIG. 2b ofthe same fiber as in FIG. 2a.

[0039] The imaging system of a fiber splicer also contains the CCDmatrices or CCD-cameras 9 which must be capable of capturing ordinarypictures and also detecting the radiation emitted by the hot fibers.FIG. 3 is a diagram showing the measured radiation spectrum from a hotfiber having a temperature of approximately 1900° C. The spectralresponse of the CCD-camera and its product by the hot-fiber radiationare also plotted in the diagram, the latter plotted curve demonstratingthat the CCD-camera is sufficiently sensitive to the light emitted byhot fibers. Thus, achromatic lens systems 7 can create sharp “hotimages” as captured by the CCD-devices 9.

[0040]FIG. 4 is diagram showing a plot of the refractive index profileof a dispersion-shifted optical fiber as measured by a near-field fibergeometry scanner. The diameter of the fiber was 125 μm and the corediameter was approximately 4 μm. The index difference between core andcladding can in the diagram be read to comprise almost 0.01. Thisslightly higher index of the core is large enough to make the corebehave as a thin cylindrical lens placed inside the fiber, the corerefracting the lateral light rays from a light source placed at somedistance behind the fiber as seen in FIG. 5. A camera system including ahigh numerical aperture objective, focused approximately at the focalpoint or more correctly at a point on the focal line of the fiber core,will thus be capable of producing a picture of the core, see thearticles cited above.

[0041] A summary of the data for a lens system suited to produce goodcold images and good hot images is given in Table 1. TABLE 1 Item TargetCold Image Target Hot Image Optical Magnifica- >8x >8x tion ConjugateLength <125 mm <125 mm Object distance >11.0 mm >11.0 mm Field of view0.25 mm 0.25 mm Numerical Aperture >0.38 — Resolution Power >40% at 400c/mm >50% at 100 c/mm (MTF) on axis Spectral Bandwidth 610-690 nm500-700 nm Distortion <0.01% <0.01%

[0042] A lens according to Table 1 having a low number of lens elements,in which the effect produced by production variation of dimensions wasminimized, was chosen to have a basic configuration analogous of aretrofocus camera lens, see FIG. 6. The objective was made of twopositive components and a negative doublet placed in the long conjugate.The conjugate length was 121 mm for an object distance of 11.1 mm.

[0043]FIGS. 7a and 7 b show plots of the Modulation Transfer Functionfor the selected lens system for two spectral intervals. Thus, FIG. 7aillustrates the resolution power of the objective for “cold images”using a background illumination from a 610-690 nm light source and FIG.7b for “hot images”, i.e. for thermal radiation from glowing fibersdetected by the CCD-camera, in the wavelength range of 500-700 nm.

[0044] An automatic fusion splicer having an imaging system capable ofcapturing both cold and hot images can be used for making a reliabledetermination of the kind of optical fibers to be spliced as will bedemonstrated hereinafter. Thereby, correct parameters to be used in thesplicing process can be selected to give splices having improvedproperties such as low insertion losses. A process for suchidentification will now be described.

[0045] Thus, FIGS. 8a, 8 b show cold and hot images respectively of atypical dispersion shifted fiber taken by the lens system as describedabove. The core is sharply visible in the hot image due to the higheremissivity of the doped core glass. The intensity (gray level) profilesof the cold and hot images are given in FIG. 9 as functions of the pixelposition in the captured images, the pixel positions corresponding to aphysical position in fiber taken along the solid lines, drawn on thepictures, extending perpendicularly to the longitudinal direction of thefiber. The center of the fiber core is visible as a central peak and thethin ring around the core can be seen as two lower peaks at each side ofthe core in the cold image profile. The fiber core can also be seen asthe central peak of the hot image profile. The image of the ring ispartly superimposed on the picture of the core and can be observed asincreased gray-level values around the core in the hot image profile.

[0046] The cold image profile and the hot image profile containinformation on the geometry and the shape of the fiber core. Thisinformation can be further processed to identify the kind of opticalfiber used. Such a process will now be described with reference to theflow diagram in FIG. 12.

[0047] In a first block 81 the focus position of the camera is set to afirst value t₁ given by $\begin{matrix}{t_{1} = {\frac{d_{c}}{D} = 0.24}} & (1)\end{matrix}$

[0048] where D is the diameter of the fiber as seen in the cold imageand d_(c) is width of the refracted illumination rays at half themaximum in the cold image as shown in FIGS. 5 and 9. This means that inthe focusing step pictures are taken when varying the focus position ofthe lens system 7 and then analyzed to determine the values d_(c) and D,until, in a captured image, the relation (1) is valid, i.e. so that theratio of the half-width value d_(c) to the apparent diameter D is equalto a predetermined value. This digital cold-image having the desiredratio is then stored in a memory of the process circuits 33. Then in ablock 83 the focus position of the camera is changed to t₂$\begin{matrix}{t_{2} = {\frac{d_{h}}{D} = 0.48}} & (2)\end{matrix}$

[0049] where d_(h) in the corresponding manner is the width of therefracted illumination rays at half the maximum in the hot image. Thus,in this case the same procedure is executed but now there is nobackground illumination, i.e. the light sources 11 are not energized,but the arc between the electrodes 9 is lit by providing a suitablecurrent to flow through the electrodes. The electrode current should belower than that used for actually fusing the ends of the fibers to eachother. The focus position is varied, pictures are captured and analyzedto find the half-width value d_(h) and the apparent diameter D, untilthe focus position gives a picture in which the condition (2) isfulfilled, i.e. in which the ratio of said quantities is equal to apredetermined value larger than that used for setting the focus positionfor cold images, e.g. equal to about twice that value (stämmer detta ???varför ???). Then this digital hot-image is stored in the processormemory.

[0050] A 3×3 mean filter is in the next block 85 applied to thegray-level values of the whole or some selected area of the stored hotimage taken for the correct focus position. In the block 87 the hotfiber profile F_(h)(x) which is a function or more exactly aone-dimensional array containing gray-level values of the picture ofpoints on a line perpendicular to the longitudinal direction of the hotimage of the fiber, is selected from the filtered area, this line thusextending in the x-coordinate direction and x representing the positionin this direction. The values F_(h)(x) are thus measured valuesrepresenting the intensity of light emitted from the correspondingpoints of the fiber. A differential array, G_(h)(x) with accentuatedspatial amplitude changes is then generated in the same block and isgiven by Eq. (3).

G(x)=F(x+1)−F(x−1)  (3)

[0051] In the same block 87 also the cold fiber profile F_(c)(x) isselected which in the corresponding way is a one-dimensional arraycontaining gray level values of the picture of points on a lineperpendicular to the longitudinal direction of the cold image of thefiber. The values F_(h)(x) are thus measured values representing theintensity of light coming from the cold fiber.

[0052] The hot grey-level array or hot fiber profile F_(h)(x) for thedispersion shifted fiber for which the images in FIGS. 8a, 8 b are takenis plotted in FIG. 10 and in the same figure its first order derivativeG_(h)(x) containing 225 pixels is also plotted. Boundaries of the coreand the ring are clearly detected as four local maximum points at themiddle of the curve of the derivative. They can be compared to the fourlocal minimum points at the middle of the cold-image profile, see FIG.9.

[0053] A range of w=100 pixels surrounding the core in the derivativearray G_(h)(x), see FIG. 9, is then selected in a block 89. The valuesof the derivative array are then compared to corresponding derivativearrays S_(n)(x) for known fibers. Before the comparison the derivativearray or arrays can be normalized and/or displaced so that thecomparison can be made in an appropriate way. In the comparison themean-square error E_(n) of the derivative array G_(h)(x) considered as adeviation of the array S_(n)(x) of each of the known fiber types iscalculated according to $\begin{matrix}{E_{n} = {\sum\limits_{x = 1}^{100}\left\lbrack {{S_{n}(x)} - {G(x)}} \right\rbrack^{2}}} & (4)\end{matrix}$

[0054] Thereafter the minimum value E_(m) of the calculated errors isdetermined in a block 91, m thus defining the fiber type most resemblingthe tested fiber. This minimum value is in a next block 93 compared to athreshold value ε₁. If the minimum value E_(m) is not below thisthreshold, the fiber is considered to be an unknown type and a signalthereof to some control device in order to e.g. showing some message ona display is sent in a block 95. If the minimum value is below thethreshold, also the intensity profile obtained from the cold image willbe evaluated.

[0055] Thus, in a block 97 the cold intensity profile F_(c)(x) islow-pass filtered by subjecting it to a 3×1 median filter. Thereupon, inthe block 99 the filtered intensity profile is normalized and displacedto some predetermined x-position and in the corresponding way, aspreviously executed in the block 89, the mean-square error E_(c) iscalculated for the filtered and normalized intensity profile as comparedto the cold image intensity profile Q_(m) of the m:th fiber type. Thecalculated mean-square error E_(c) is in a block 101 compared to athreshold value ε₂. If the result of the comparison is that themean-square error is smaller than the threshold value, the tested fiberis determined to be type m and a signal thereof is sent in a block 103to some control device for e.g. appropriately setting welding parametersand/or displaying some message. If it is determined in the block 101that the mean-square error is not smaller than the second thresholdvalue the tested fiber is decided not to be any of the known types andthen a signal thereof is sent in a block 105 to some control device.

[0056]FIG. 12a is a diagram of the refractive index profile of the core,FIG. 12b is a diagram showing measured pixel gray level values Q_(m)(x)and S_(h)(x) for cold and hot fibers respectively and the derivativearray G_(h)(X) of the hot profile and FIGS. 12c-12 d show hot and coldimages of a depressed cladding, highly doped dispersion shifted fiber.FIGS. 13a-13 d are the corresponding diagrams and pictures for an erbiumdoped fiber, FIGS. 14a-14 d for a standard single-mode fiber, SMF-28,FIGS. 15a-15 d a for large effective area none-zero dispersion shiftedfiber, FIGS. 16a-16 d for a multi-mode fiber, and FIGS. 17a-17 d for apure silica core single-mode fiber.

[0057] While specific embodiments of the invention have been illustratedand described herein, it is realized that numerous additionaladvantages, modifications and changes will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, representative devices andillustrated examples shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents. It is therefore to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin a true spirit and scope of the invention.

1. A method of determining the type of an optical fiber, the methodcomprising the steps of: heating a region of the optical fiber to such atemperature that an observable amount of light is emitted, recording thelight emitted during the heating as a first picture, determining fromthe first picture a first light intensity profile comprising values ofthe intensity of light emitted from all points in a line substantiallyperpendicular to the longitudinal direction of the fiber, as viewed inan observation direction, as a function of the position along the line,and analyzing the determined light intensity profile, characterized inthat in the step of analyzing, the derivative of the determined firstlight intensity profile is calculated and compared to the derivative offirst light intensity profiles previously determined for a plurality ofoptical fibers of known different types.
 2. A method according to claim1, characterized in that in the step of determining a first lightintensity profile, a local mean is calculated for each point in thefirst picture to produce a smoothed picture.
 3. A method according toclaim 1, characterized in that in the step of analyzing, the determinedfirst light intensity profile is low-pass filtered before calculatingthe derivative.
 4. A method according to claim 1, characterized in thatin the step of analyzing, the derivative of only a central portion ofthe determined first light intensity profile is calculated and comparedto the derivative of corresponding central portions of the first lightintensity profiles previously determined for the plurality of opticalfibers of known different types.
 5. A method according to claim 1,characterized by the additional steps of: taking a second picture of aregion of the optical fiber in an unheated state using a high-resolvingoptical system to produce in the picture an image of the core of theoptical fiber, determining from the second picture a second lightintensity profile comprising values of the intensity of light emittedfrom all points in a line substantially perpendicular to thelongitudinal direction of the fibre, as viewed in an observationdirection, as a function of the position along the line, and comparingthe determined second light intensity profile to second light intensityprofiles previously determined for the plurality of optical fibers ofknown different types, and evaluating the results of the comparing ofthe first and second light intensity profiles to find the type of knownoptical fiber which most resembles the optical fiber for which thepictures have been taken.
 6. A method according to claim 5,characterized in that in the step of determining a second lightintensity profile, directly after the determining, a local mean iscalculated for each point in the determined second light intensityprofile to produce a smoothed second light intensity profile which isthen compared.
 7. A method according to claim 5, characterized in thatin the step of determining a second light intensity profile, directlyafter the determining, the determined second light intensity profile islow-pass filtered before comparing.
 8. A method of splicing two opticalfibers to each other, comprising the steps of: aligning one end of afirst one of the optical fibers with one end of a second one of theoptical fibers, moving the end of the first optical fiber to place theend surface of this end at the end surface of the end of the secondoptical fiber, heating a region of the ends of the optical fibers at theend surfaces to make them be fused to each other, and allowing theregion to cool, characterized by the additional steps of: determiningthe types to which the optical fibers belong by the method according toany of claims 1-7, setting at least one of physical parameters used inthe step of moving and/or the step of heating as determined in advancefor a splice of optical fibers of the determined types.
 9. A methodaccording to claim 8 when in the heating step, an electric arc is used,characterized in that in the step of setting physical parameters, theintensity of electrical current flowing between electrodes is set.
 10. Amethod according to claim 8, characterized in that in the step ofsetting physical parameters, at least one of the following is made:setting the duration of heating of the region, setting the intensity ofheating of the region, setting in the step of moving an overlapdistance.
 11. A device for determining the type of an optical fiber, thedevice comprising: heating means for heating a region of the opticalfiber to such a temperature that an observable amount of light isemitted, an optical system for imaging the region of the optical fiber,light sensitive means for recording light emitted when heating theregion and imaged by the optical system as a first picture, determiningmeans connected to the light sensitive means for determining from thefirst picture a first light intensity profile comprising values of theintensity of light emitted from all points in a line substantiallyperpendicular to the longitudinal direction of the fiber, as viewed inan observation direction, as a function of the position along the line,and analyzing means connected to the determining means for analyzing thedetermined light intensity profile, characterized in that the analyzingmeans comprise: calculating means for calculating the derivative of thedetermined first light intensity profile, comparing means connected tothe calculating means for comparing derivatives of light intensityprofiles previously determined for a plurality of optical fibers ofknown different types to the calculated derivative of the first lightintensity profile, and decision means connected to the comparing meansfor deciding, in the case where a signal provided by the comparing meansindicates that for one of the optical fibers of known different types,the calculated derivative deviates least from the derivative of thelight intensity profile of said one of the optical fibers, that theoptical fiber is the same type as said one of the optical fibers.
 12. Adevice according to claim 11, characterized in that the comparing meansare arranged to calculate values representing the differences betweenthe derivatives of the light intensity profiles for the plurality ofoptical fibers of known different types and the calculated derivative ofthe first light intensity profile, and the decision means are arrangedto compare the calculated value representing the differences between thederivative of the light intensity profile for said one of the opticalfibers of known different types and the calculated derivative of thefirst light intensity profile to a threshold value and only deciding theoptical fiber to be same type as said one of the optical fibers in thecase where the said value is not greater than the threshold value.
 13. Adevice according to claim 11, characterized in that the optical systemis high resolving allowing, for a correctly setting, imaging the regionof the optical fiber in an unheated state to produce in the producedsecond picture an image of the core of the optical fiber, the lightsensitive means recording also the second picture, the determining meansare arranged to determine from the second picture a second lightintensity profile comprising values of the intensity of light emittedfrom all points in a line substantially perpendicular to thelongitudinal direction of the fibre, as viewed in an observationdirection, as a function of the position along the line, and thecomparing means are arranged to compare the determined second lightintensity profile to second light intensity profiles previouslydetermined for the plurality of optical fibers of known different types,and the decision means are arranged to evaluate the results of thecomparing of the derivatives of the first light intensity profiles andthe second light intensity profiles to find the type of known opticalfiber which most resembles the optical fiber for which the first andsecond pictures have been recorded.
 14. A splicer for splicing twooptical fibers to each other, comprising: aligning means for aligningone end of a first one of the optical fibers with one end of a secondone of the optical fibers, moving means for moving the end of the firstoptical fiber to place the end surface of this end at the end surface ofthe end of the second optical fiber, and heating means for heating aregion of the ends of the optical fibers at the end surfaces to makethem be fused to each other, characterized by a device for determiningthe types to which the optical fibers belong according to any of claims11-13, and control means connected to the device for determining thetypes and arranged to command the device to determine the types of theoptical fibers to be spliced before malting the splice, to then find avalue of at least one of physical parameters adapted to the determinedtypes and control the respective one of the aligning, moving and heatingmeans to use the found value.
 15. A method of automatically setting anoptical system for imaging an optical fiber, characterized by the stepsof: varying the distance between the optical fiber and the opticalsystem to give pictures, determining from each of the pictures a lightintensity profile comprising values of the intensity of light emittedfrom all points in a line substantially perpendicular to thelongitudinal direction of the fiber, as viewed in an observationdirection, as a function of the position along the line, and analyzingthe determined light intensity profile to find in the picture a measureof the apparent diameter of the optical fiber and a measure of the widthof a central peak, calculating the ratio of the measures and comparingit to a predetermined value, and setting the distance for a correctimaging so that the ratio substantially agrees with the predeterminedvalue.